Vacuum deposition is a common method for coating metals such as aluminum, copper, zinc, and tin onto various substrates of metal, glass, and plastic. The metal is typically vaporized by means of electric resistance heating in a metallic or ceramic vessel generally referred to in the art as a "boat." The boat is connected to a source of electrical power within an evacuated chamber and heated to a controlled operating temperature sufficient to cause a metal charge placed in contact with the boat to vaporize. The metallizing temperature, that is, evaporation temperature controls the rate of evaporation of the metal charge which, in turn, determines product production.
There is a current need in the industry to increase the useful life of the boat so as to increase product production and reduce the overall cost of the deposition process without necessarily increasing the rate of metal evaporation from current rates of metal evaporation. Evaporation rates for aluminum of about 1.8 to 2.5 gms/min./in.sup.2 or lower are now conventional and are achieved at evaporation temperatures at or below 1500.degree. C. At this evaporation rate, an intermetallic composite--composed of either a binary intermetallic composition of titanium diboride and boron nitride or a ternary composition of titanium diboride, boron nitride, and aluminum nitride--has been effectively used to produce a high-density, relatively strong boat. If the evaporation temperature exceeds 1500.degree. C., the rate of reaction of the aluminum metal with the boat increases to adversely reduce the useful life of the boat. A causal relationship exists between the rate of reaction of aluminum and the boat which affects the useful life of the boat and is primarily attributable to the composition of the boat and its density. The useful life of the boat is also affected by the degree of wetting of the boat upon vaporization of the aluminum metal charge. Heretofore it has not been possible to increase the useful life of the boat without substantially degrading boat performance.